1. Field of the Invention
The present invention relates to an image processing apparatus and method of adjusting color balance, and a storage medium.
2. Related Background Art
Conventional methods of adjusting a color balance shift of a color image or a color input/output device are roughly classified into two methods. In one method, when a color image is represented by the RGB or CMY color space, the color balance is adjusted on the RGB or CMY color space. In the other method, the RGB or CMY color space is temporarily converted into color spaces expressing certain brightness and chromaticity, and the color balances for brightness and chromaticity are independently adjusted in the corresponding color spaces. The adjusted brightness and chromaticity are converted back into the original color space. The first and second methods associated with RGB image data will be described below.
The first adjustment method is expressed by:R′=R+α  (1)G′=G+β  (2)B′=B+γ  (3)where α, β, and γ are arbitrary positive or negative values.
By changing the values α, β, and γ, the color balance of an object image can be adjusted.
In the second method, for example, the RGB color space is converted into the L*a*b* color space defined by CIE:RGB→L*a*b* (where L* represents brightness and a* and b* represent chromaticity) and adjusted. The following conversion processing is performed on the L*a*b* space:L*′=L*+α  (4)a*′=a*+β  (5)b*′=b*+γ  (6)where α, β, and γ are arbitrary positive or negative values.
By converting L*′ a*′ b*′ back into R′ G′ B′, L*′ a*′ b*′→R′ G′ B′
color balance adjustment processing can be realized.
However, in prior art 1, since appropriate adjustment amounts are added to or subtracted from R, G, and B, respectively, the brightness and chromaticity are simultaneously adjusted.
Even when the operator wants to adjust only the chromaticity, the brightness is also adjusted. In addition, in color balance adjustment on the RGB color space, the operator cannot intuitively recognize the destination and amount of color balance adjustment for the entire image, so color balance adjustment directly reflecting the operator's intention for adjustment cannot be performed. Furthermore, since predetermined RGB color balance adjustment is performed on the RGB color space, which is not a uniform color space, independently of the brightness value of object pixel data, the apparent adjustment amount changes depending on the brightness.
In prior art 2, since color balance is adjusted after the RGB color space is converted into spaces expressing the brightness and chromaticity, the brightness and chromaticity can be independently adjusted. This allows direct reflection of the operator's intention for adjustment. However, conversion from the RGB color space into the color spaces expressing the brightness and chromaticity requires a long time.
In prior arts 1 and 2, since the operator cannot recognize the position of a set adjustment amount in the entire adjustment space, he/she cannot know the degree of adjustment based on the adjustment amount defined by himself/herself.